Hydraulic fracturing is a well stimulation technique designed to enhance the production of hydrocarbons from a well. This technique creates highly permeable conductive fractures or channels in a producing formation surrounding a well bore. The fracturing process typically involves injecting fluids down a perforated well bore at sufficient rate and pressure to rupture the formation, thereby creating one or more fractures in the reservoir rock. High pressure pumping is then continued to extend the fractures generally radially outwardly from the well bore. In order that the fractures which are created do not close once the fracturing pressure is released, the fracturing fluid typically contains a particulate material, known as a propping agent or proppant, which is dispersed throughout the fractures. Once the pressure is released and the fracturing fluid leaks off into the formation, the fractures close about the proppant. The captured proppant maintains the fracture opening by resisting forces tending to close the fracture. The proppant laden fractures produce highly conductive channels which facilitate the flow of desired fluids from the formation into the well bore.
In the past, propped hydraulic fracturing was not commonly used to stimulate formations having high contrast in permeability. Often sands have a fining upward sequence. The upper part of the sands have a low permeability and the lower part has a very high permeability. For example, the upper part of the formation may have permeabilities in the range of 20 to 300 mD, while the lower part has permeabilities ranging from 1,000 to 10,000 mD.
During production, the high permeability lower zones of the formation may be swept with water to maximize hydrocarbon extraction. Meanwhile, the concentration of hydrocarbons can remain high in relatively low permeability zones. Fracture stimulation in such an environment often props open both the oil and water-bearing portions of the formation, which results in water production overwhelming the oil or gas production.
Accordingly, there is a need for a hydraulic fracturing technique which enhances the conductivity and production of oil and/or gas from lower permeability hydrocarbon bearing zones while minimizing increases in the conductivity of connected zones having higher permeability and containing higher concentrations of water. The present invention addresses this need.